1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a double enclosure top nozzle subassembly allowing improved utilization of space for accommodating greater thermal growth and burnup of fuel rods of the fuel assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies which receive support and alignment from upper and lower transversely extending core support plates. The upper and lower core support plates are directly or indirectly attached to a support barrel which surrounds the entire core and extends between the ends thereof.
Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by a plurality of grids spaced along the fuel assembly length and attached to the control rod guide thimbles. The guide thimbles extend slightly above and below the ends of the fuel rods. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles to thereby form an integral fuel assembly.
The fuel assemblies are arranged vertically resting on the lower core support plate. To facilitate handling and installation, the fuel assemblies are generally not secured to the upper and lower core support plates.
Temperatures at various times within the core may vary greatly, such as, from cold shutdown to normal operating conditions. Also, different materials exhibit different thermal growth characteristics. Since the materials of fuel assembly components are generally different than those used in the core support barrel and undergo greater thermal expansion, the resulting increase in length of the fuel assemblies in the axial or vertical direction must be accommodated. For this reason also, the fuel assemblies are not usually attached to the upper and lower core plates but rather are supported in a manner which permits some relative motion therebetween.
The axial thermal expansion differential between the fuel assemblies and the core support barrel has been accommodated by insuring that the axial spacing between the upper and lower core support plates is somewhat greater than the axial length of the fuel assemblies. Normally, this is accomplished by providing an axial gap between the top of the fuel assemblies and the upper core support plate. However, the presence of the gap can result in upward lifting of the fuel assemblies due to the hydraulic forces produced on the fuel assemblies in the upward direction by coolant flow. Thus, fuel assemblies have also employed hold-down devices with the top nozzles to prevent the force of upward coolant flow from lifting the fuel assemblies into damaging contact with the upper core support plate, while at the same time allowing for changes in fuel assembly length due to core-induced thermal expansion and the like. Representative to the prior art fuel assemblies with hold-down devices are those disclosed in Klumb et al U.S. Pat. No. 31,583, Anthony Pat. No. 4,078,967, Gjertsen et al U.S. Pat. No. 4,534,933, Wilson et al Pat. No. 4,620,960 and Wilson et al U.S. Pat. No. 4,670,213.
As mentioned previously, the guide thimbles of fuel assemblies extend slightly above and below the ends of the fuel rods. Thus, the top and bottom nozzles of fuel assemblies secured at the opposite ends of the guide thimbles likewise are spaced above and below the fuel rod ends. This space between the opposite ends of the fuel rods and adjacent portions of the top and bottom nozzles accommodates increase in length of the fuel rods due to thermal growth as fuel rod burnup occurs during normal reactor operation.
With improvements in various aspects of fuel assembly design, it has become feasible to increase the allowable burnup of the fuel rods. This increase in burnup is desirable because it decreases the frequency of plant shutdowns and the buildup of spent fuel. However, to permit the fuel rods to operate to a higher burnup, an increase of approximately 0.5 inch minimum in fuel rod length is necessary due to extra thermal growth. This necessitates an increase in the space between the adapter plates of the top and bottom nozzles to accommodate this additional fuel rod growth. At the same time, there still must be enough space between the top plate and adapter plate of the top nozzle to allow the handling equipment of the core to get between the plates and latch onto the underside of the top plate of the top nozzle.
Heretofore, there has not been enough room between the adapter plates of the top and bottom nozzles to permit the 0.5 inch growth in fuel rod length. A proposed solution to this problem is the expandable top nozzle subassembly of the above cross-referenced U.S. Pat. No. 4,986,959. This proposed design provides an extension of fuel assembly length sufficient to enable the fuel assembly to accommodate fuel rod growth in excess of one inch, allowing a reactor to operate at a higher burnup rate. Also, where extremely high burnups are not required, the mechanical duty of the fuel rods can be reduced significantly.
The proposed expandable top nozzle subassembly design of the cross-referenced application U.S. Pat. No. 4,986,959 makes use of "dead space" already existing in the conventional top nozzle between its top plate and bottom adapter plate. This space is characterized as "dead" since it is only used during installation and removal of the fuel assembly and not during operation of the core. The expandable (and compressible) top nozzle subassembly of the cross-referenced application U.S. Pat. No. 4,986,959 thus provides for the additional fuel rod growth space needed, while continuing to allow the use of current handling systems and thus eliminating potential costs to customers in design changes.
Basically, the expandable top nozzle subassembly of the cross-referenced application U.S. Pat. No. 4,986,959 includes a lower adapter plate, and an upper structure having a top plate and a sidewall enclosure rigidly connected to and depending from an outer peripheral edge of the top plate. The lower adapter plate is disposed below the top plate of the upper structure and within the sidewall enclosure thereof. The lower adapter plate and sidewall enclosure are slidably movable relative to one another so as to move the top plate away from and toward the lower adapter plate between expanded and compressed positions of the top nozzle subassembly.
When the expandable top nozzle subassembly is in the compressed position, the lower peripheral edge of the sidewall enclosure is disposed below the lower adapter plate and near the upper ends of the outermost ones of the fuel rods. The close proximity of the lower peripheral edge of the sidewall enclosure to the fuel rods has raised concern that contact will occur between them upon their vibration during reactor operation and result in fretting of the claddings of the fuel rods.
Consequently, a need still remains for an alternative design of a top nozzle that will accommodate extra fuel rod thermal growth while avoiding contact with the fuel rods and without impairing the handling capability of the core equipment currently in use.